Suppression of diffraction in deep-inelastic scattering on nuclei and dynamical mechanism of leading twist nuclear shadowing

A bstract Using the leading twist approach (LTA) to nuclear shadowing, we calculate the ratios of diffractive and usual parton distributions for a heavy nucleus (Pb) and the proton, RA/p=(fi/AD(3)/fi/A)/(fi/pD(3)/fi/p) R A / p = f i / A D 3 / f i / A / f i / p D 3 / f i / p , for coherent and summed (coherent plus quasi-elastic) nuclear deep-inelastic scattering. We find that R A/p ≈ 0.5 − 1 for quarks as well as for the ratio of the diffractive and total cross sections [(dσdiff/d MX²)/σₜₒₜ]eA/[(dσdiff/d MX²)/σₜₒₜ]ₑₚ dσ diff / dM X 2 / σ tot eA / dσ diff / dM X 2 / σ tot ep and R A/p ≈ 0.5 − 1.3 for gluons in a broad range of x , including the kinematics of the Electron-Ion Collider, which reaffirms the difference from the nuclear enhancement of R A / p predicted in the gluon saturation framework. We demonstrate that the magnitude of R A / p is controlled by the cross section of the interaction of hadronic fluctuations of the virtual photon with target nucleons, which explains an enhancement of R A / p in the color dipole model and its suppression in LTA. We argue that the black disk limit corresponds to R A / p = 1 and RA/pᶜᵒʰ R A / p coh = 0.86 for the summed and coherent scattering, respectively. Relying on an intuitive definition of the saturation scale, we show that the ratio of the saturation scales of a heavy nucleus and proton QsA²(b)/Qₛₚ²(b)≈ 1 Q sA 2 b / Q sp 2 b ≈ 1 at small impact parameters b due to the strong leading twist nuclear shadowing and diluteness of the nuclear density.